The use of neoadjuvant chemotherapy prior to resection or radiation therapy has contributed to dramatic improvement in survival of patients with pediatric malignancies. Precise clinical and radiologic methods of evaluating the primary lesion at diagnosis and assessing response to chemotherapy are necessary to select appropriate sites for biopsy, plan surgical procedures and estimate prognosis. Standard definitions of response rely on changes in uni- or bidimensional measurements and do not take into account the relative amounts of residual viable malignant disease and necrosis within the tumor mass. The development of more accurate definitions of response based on imaging technologies which are capable of distinguishing active disease from non-malignant tissue is therefore an important management goal. This research will be performed within the framework of existing institutional protocols for treatment of Ewing sarcoma, rhabdomyosarcoma, and related malignancies which have a total annual accrual of approximately 25 patients. We will perform routine CT, MR and nuclear imaging of the primary tumor and will investigate volume measurements, dynamic contrast-enhanced MR imaging (DEMRI) and Thallium scintigraphy in assessing response to neoadjuvant chemotherapy prior to radiation or surgery. Our investigations will include: A. use of interactive displays for calculation of tumor volumes from three dimensional magnetic resonance (MR) imaging data after transfer to a Sun Unix workstation. B. Use of dynamic gadolinium-enhanced MR imaging (DEMRI) for measuring the rate of contrast-enhancement in multiple regions of a representative tumor section, displayed in a pixel-by-pixel manner on the Sun Unix workstation. Data will be used to assess response to chemotherapy. C. Use of thallium-201 nuclear imaging as a method of assessing tumor viability by quantitating lesional uptake before and after neoadjuvant chemotherapy. Changes in volume and, results of DEMRI and thallium-201 scans will be compared to standard response criteria based on clinical findings, routine CT/MR imaging, and pathologic evidence of tumor activity on biopsy or histologic maps of tumor activity in resected specimens.